In an era that emphasizes green technology, there is a need for finding new ways to save and reuse energy, while also making it affordable to do so. Energy harvesting refers to a process of capturing energy from external sources comprising, for example, sunlight, thermal energy, wind, kinetic energy, hydraulics, etc. Energy that is harvested from different sources is typically bountiful, and is present regardless of whether energy harvesting takes place. The harvested energy is typically converted to electricity to power electronic devices, for example, wireless autonomous devices used in wearable electronics and wireless sensor networks. Since energy harvesting does not depend on batteries or power sockets, the harvested energy is used as a power source in multiple different industries and for powering portable electronic devices. For example, users can use the harvested energy to charge portable electronic devices such as smartphones without the need for connecting their smartphones to a power socket, thereby allowing the users to charge their smartphones on the go. Other devices, for example, communication radios and flashlights can also benefit from energy harvesting technologies in locations such as underground mines, deserts, and remote areas, where power sources are unavailable.
Conventional generators produce electricity from random, ambient vibrations to power a device, for example, a wristwatch, a pacemaker, or a wireless sensor. Some energy harvesting devices generate renewable electric power from arbitrary, non-periodic vibrations. Non-periodic vibrations are obtained, for example, from traffic driving on bridges, machinery operating in industries, and humans moving their limbs. In a research experiment, a generator that harnesses energy from nearby vibrations using piezoelectric materials was created. The piezoelectric materials create a charge when stressed. The piezoelectric materials allow each generator of one cubic centimeter in volume to generate a power of, for example, about 0.5 milliwatts, which can potentially be used to drive small autonomous devices, for example, pacemakers. The conventional energy harvesting systems that use piezoelectric materials generate insufficient power to power a standard portable electronic device. Moreover, the piezoelectric materials are expensive. In another research experiment, vibration-to-electricity converters that use microelectromechanical systems (MEMS) fabrication technology with an output power density of, for example, about 116 μW/cm3 were designed. However, the MEMS based energy harvester system is expensive and generates low power.
Another conventional energy harvester system uses micro-electrostatic vibrations to generate electricity. The reduction in size and power consumption of complementary metal-oxide semiconductor (CMOS) circuitry has led to research based on wireless sensor networks. Proposed networks comprise thousands of small wireless nodes that operate in a multi-hop fashion, replacing long transmission distances with multiple low power and low cost wireless devices. The result is a creation of an environment that responds to its inhabitants and ambient conditions. Wireless devices being designed and built for use in such an environment typically run on batteries. However, as the networks increase in number and the devices decrease in size, the replacement of depleted batteries is not practical. The cost of replacing batteries in a few devices that make up a small network about once a year is feasible. However, the cost of replacing batteries in thousands of devices annually, some of which are in areas difficult to access, is not practical. Another approach would be to use a battery that is large enough to last the entire lifetime of a wireless sensor device. However, a battery large enough to last the lifetime of the wireless sensor device would dominate the overall system size and cost, and thus is not practical. There is a need for alternative methods of powering devices that make up wireless networks.
In another experimental research study, a brushless direct current (BLDC) motor was created. The BLDC motor is a robust machine which has applications over a wide range of power and speed in different shapes and geometry. The BLDC motor or generator consists of two magnetically dependent stator and rotor sets or layers, where each stator set comprises nine salient poles with windings wrapped around them, while the rotor comprises six salient poles. A magnetic field passes through a guide to the rotor, then to the stator, and finally completes its path via a housing of the BLDC motor. This is a three phase motor or a three phase generator and every stator and rotor pole arc is about 30°. In this research study, a power electronic converter was also presented. This topology provides bidirectional control of a current for each motor phase independently. A control scheme permits the BLDC motor to operate with any number of phases at any time. In this converter, four power switches in the form of a bridge connection for each motor phase was utilized and therefore, the BLDC motor was operated by switching different sequences for the current direction in each motor phase winding. This converter also offers a choice of having any number of phases to be activated at any time. A prototype motor or generator and a drive circuit were built and tested in a laboratory and the numerical and experimental results were presented. Due to the ruggedness of the proposed motor or generator in comparison with the conventional and BLDC motors used for automobile applications, the proposed motor or generator was applicable for use as an integrated motor generator for a hybrid vehicle.
Another research study provided possible strategies to increase an operational frequency range for vibration based micro-generators. Most vibration based micro-generators are spring-mass-damper systems that generate maximum power when a resonant frequency of the vibration based micro-generator matches a frequency of an ambient vibration. Any difference between these two frequencies results in a significant decrease in the generated power, which restricts the capability of resonant vibration generators in real applications. Possible solutions comprise, for example, periodic tuning of the resonant frequency of the generator to match the frequency of the ambient vibration at all times or widening the bandwidth of the generator. Periodic tuning is achieved using mechanical or electrical methods. Bandwidth widening is achieved, for example, using a generator array, a mechanical stopper, nonlinear or magnetic springs, or bi-stable structures. Tuning methods are classified into intermittent tuning and continuous tuning. In the intermittent tuning method, power is consumed periodically to tune the generator. This scenario presents a comprehensive review of principles and operating strategies for increasing the operating frequency range of vibration based micro-generators.
Energy harvesting generators are typically used as inexhaustible replacements for batteries in low power wireless electronic devices. Ambient motion is one of the main sources of energy for harvesting, and a wide range of motion powered energy harvesters are proposed or demonstrated. Another conventional energy harvester system generates electricity from mechanical energy. Ambient mechanical vibrations move a magnet which is attached to a harvester frame. A moving magnetic field induces an electromotive force in a coil placed outside of the harvester frame. The energy harvester system generates electrical power of up to a few milliwatts. Most energy harvester systems utilize expensive materials, for example, piezoelectric materials to generate electricity. Moreover, the power generated by these energy harvester systems is, for example, about 50 milliwatts to about 100 milliwatts, which is not sufficient to power smartphones or other portable devices.
The harvested energy from different energy harvesting systems such as those discussed above is often used to drive an alternating current (AC) generator to produce electricity. Most generators used in industries employ two coils, where one coil is used as an electromagnet and the other coil is used for inducing an electric current. That is, one of the coils spins and the other coil remains stationary. The conventional AC generators have either a rotating electromagnet or a rotating coil. As a result, the two coils, that is, the coil of the electromagnet and the coil of current induction cannot share the same iron core and the magnetic field generated by the coil of the electromagnet can only partially travel to the other coil for the induction of electric current. A major portion of the magnetomotive force used to induce the magnet field is thus wasted. Because the two coils cannot share the same iron core, the induced magnetic field has to pass through air to enter the other iron core, losing strength in the process. The induced magnetic field by the coil of the electromagnet cannot entirely travel to the coil of current induction to induce electric current and as such, the electromagnet has to produce a stronger magnetic field by using more magnetizing current.
Hence, there is a long felt but unresolved need for a dynamic system that generates electricity from a changing magnetic field. Moreover, there is a need for a dynamic system that uses only one iron core for both the coil of the electromagnet and the coil of current induction, where both the coils are stationary and the changing magnetic field is created by turning on/off the magnetizing current.